Instincts.—In the typical instinct there is a series of “chain reflexes” in which one step determines the next until mechanically the whole gamut of changes is run to the last step. It is characteristic of a purely instinctive act that an animal performs it without practise, without instruction, and without reason. Moreover, all of the same kind of animals tend to perform the act in the same way. But with instincts, as with tropisms, the physiological state of the organism must be regarded. For instance, the instinctive reactions of an animal sated with food or hungry will be different.
Adjustability of Instincts Opens the Way for Intelligent Behavior.—As we progress in the scale of animal life this adjustability of instincts to new conditions comes more into evidence. While prescribed in the main by internal impulse the carrying out of the action is capable of some adaptability to circumstances. And in proportion as this adaptability releases the organism from a blind rigid working-out of a predetermined end, there is opened up the possibility of intelligent behavior; that is, of modification of the instinctive behavior by individually acquired experience.
While the generation of instinctive impulses still occurs it is left more for individual experience to teach discrimination between ends. But we can not escape a fundamental structural mechanism, for with this new capacity of educability must come new structural mechanisms in the nervous system and this must be as faithfully reproduced in each individual as is the basis for any other nervous response. How low in the scale of animal life animals can profit from their experiences to the extent that their future conduct is conditioned thereby is not known. Some would place it as far back as the protozoa, others would not. Where such modification of behavior is possible there must be some mechanism for the storage of impressions in the form of what we term memory.
Modification of Habits Possible in Lower Animals.—Among invertebrates such animals as crayfish will acquire new habits, or rather will modify old ones. Even as lowly an organism as the starfish can have changes of habit thrust on it. When a starfish is placed upon its back it rights itself by means of its arms or rays. Professor Jennings found that in a given individual the tendency was always to employ certain rays for this rather than others. However, by preventing the use of the rays customarily employed, he found that the animal would use a different pair and that ultimately in this way it could be trained into the habit of using this pair of rays even when restrained in no way. One starfish which was given one hundred eighty such lessons in eighteen days after an interval of seven days still retained the new habit; young individuals were found to be more easily trained than old ones.
Some Lower Vertebrates Profit by Experience.—Among vertebrates it is known that those as low in organization as fish will profit by experience. They will learn to come for food at a regular time and apparently learn more or less to appreciate the presence of certain obstacles with which they have had unsatisfactory experiences. Professor Sanford sums up what he believes are the limitations of the piscine mental organization as follows: “No fish is ever conscious of himself; he never thinks of himself as doing this or that, or feeling in this way or that way. The whole direction of the mind is outward. He has no language and so can not think in verbal terms; he never names anything; he never talks to himself; as Huxley says of the crayfish, he ‘has nothing to say to himself or any one else.’ He does not reflect; he makes no generalizations. All his thinking is in the present and in concrete terms. He has no voluntary attention, no volition in the true sense, no self-control.”
Rational Behavior.—Finally, however, out of these first dull glimmerings of intelligence as exemplified in the higher invertebrates and the lower vertebrates, which can modify behavior as the result of experience, come the still higher factors so dominant in man, of rational behavior. This higher mental process can realize the end to be reached and can deliberate on the means to be employed. By means of his reason man can overcome difficulties in advance by “thinking” out suitable schemes of action. Some naturalists believe that man stands alone in possessing the power to reason, although others believe that some of the other mammals, notably the other primates, possess the same attribute although in a much less degree.
Conceptual Thought Probably an Outgrowth of Simpler Psychic States.—Is the capacity for such conceptual thought, however, which appears as the final efflorescence of complex neural activity something entirely new? Most students of comparative psychology maintain that it is not. Just as one kind of an instinct frequently grows out of another, so has this grown out of the complex of psychic states which preceded it. It apparently is the product of the increasing awareness on the part of animals of their neural processes and the outcome of these processes, which becomes more and more prominent as we ascend the scale of animal life. With the advent of associative memory the mind comes more and more to deal with attributes of objects instead of merely with each single concrete object as it presents itself, and these attributes being common to many objects, come to represent definite ideas which can be manipulated by the mind. Language, of course, has been an indispensable aid to man in this regard, for words become descriptions of facts and symbols of concepts, and thereby allow of abstract thought.
The Capacity for Alternative Action in High Animals Renders Possible More Than One Form of Behavior.—With this modification of instinct by experience made possible, there comes at the same time, of course, the capacity for a rational instead of a purely instinctive behavior. This very capacity for alternative action opens up many new possibilities of behavior and together with the well-known fixative effects of habit, also the opportunity of permanently establishing certain ones. Thus it is obvious that a behavior toward which in a strict sense there can not be said to have been an original specific tendency, can be developed. What was present in the first place was only a general possibility of the development of any one of several types of behavior. The final choice of the alternatives together with repetition makes it the habitual behavior of the individual. Of course it can be urged that if the selection of the type of behavior is left to the individual then the latter will operate automatically toward the various impulsions of its neural make-up and one path will be followed because of stronger inclination in that direction, so that the whole procedure is in the end the mere operation of an automaton. But however this may be in the individual left to itself, the fact is in man that the young individual is never left to itself and in the nature of things can not be, so that without entering into this troubled pool of controversy regarding freedom of the will, I wish merely to point out that the possibility of more than one form of behavior exists and that if one is more desirable than the others then this one can be chosen by the ones responsible for the training of the young individual and clenched fast by the agency of habit.
Intelligence, reason and habits, however, no less than instincts and tropism must have neural as well as psychical existence and we can not escape therefore the underlying mechanism.
The Elemental Units of the Nervous System Are the Same in Lower and Higher Animals.—It is interesting to note that the fundamental neural mechanism which underlies the mental processes of higher animals is not essentially different from that which serves in lower forms. Although as animals become more complex their nervous systems have become proportionately larger and incomparably more intricate, still all the changes have been rung on the same basic neural unit, the neuron or nerve-cell (Fig. 32A, [p. 209]). The higher nervous system differs from the lower in the number, in the specializations and in the associations of these units rather than in possessing something of entirely different elemental structure.